Microsoft has pushed Windows 10 Mobile build 10586.456 to Windows Insiders in the Release Preview ring today.
As the build number indicates, this latest release is not from the Redstone development branch that will eventually produce the Anniversary Update, which will be available on August 2. Rather, it's a cumulative update to the Current Branch, and will likely be released soon for all Windows 10 Mobile users not on the Windows Insider Program.
The most recent Current Branch build in general usage is 10586.420, which was released earlier this month.
Unfortunately, Microsoft rolled out build 10586.456 to the Release Preview ring today without publishing a changelog for it, so it's not yet clear exactly what the new build has to offer. Cumulative updates rarely bring new features, though - so don't get your hopes up on that front - but it's likely to include plenty of bug fixes and improvements.
You can find out more about the Release Preview ring and the different Windows 10 development branches here.
Update: Microsoft has said that Windows 10 build 10586.456 is also available for PCs on the Release Preview ring, in addition to phones.